(1) Field of the Invention
The invention relates to a method of metallization in the fabrication of integrated circuits, and more particularly, to a method of forming air gaps between metallization in the manufacture of integrated circuits.
(2) Description of the Prior Art
One of the methods of reducing RC delay in aluminum interconnects and copper damascene interconnects is to introduce low dielectric constant materials. In the aluminum interconnects, introduction of such low dielectric constant materials reduces predominantly the intra-metal capacitance. The most ideal candidate for the low dielectric constant material is air. In copper damascene interconnects, the integration concerns of organic low dielectric constant materials (e.g. FLARE) are being intensively addressed. In the meantime, the fabrication of dual and single damascene interconnects is being done using undoped silicon dioxide or doped silicon dioxide (e.g. FSG). One method of further lowering the dielectric constant of the silicon dioxide is to introduce air in the form of air plugs into the dielectric.
U.S. Pat. Nos. 5,324,683 and 5,510,645 to Fitch et al disclose a method of forming air gaps between metal lines by etching away spacers within the space between metal lines and covering the space with a non-conformal material which leaves air gaps where the spacers were. U.S. Pat. No. 5,461,003 to Havemann et al teaches filling gaps between metal lines with a polymer, depositing a porous dielectric layer over the polymer, then removing at least a portion of the polymer through the porous dielectric layer using an oxygen plasma treatment to leave air gaps between the metal lines. Havemann's polymer is a xerogel which may require a long processing time and may not be manufacturable. Control of porosity may not be reliable. U.S. Pat. No. 5,750,415 to Gnade et al teaches a porous silica precursor film over an underlying liquid material between metal lines. The liquid is removed through the porous film by evaporation leaving air gaps between the metal lines. U.S. Pat. No. 5,950,102 to Lee forms interconnects through multiple photoresist layers, then removes all of the photoresist leaving free-standing interconnects surrounded by air. U.S. Pat. No. 5,407,860 to Stoltz et al teaches depositing a spin-on-glass layer over metal lines having sidewall spacers. Air gaps are left between the spacers.